“This is just the tip of the iceberg,” says Andrew Glikson from The Australian National University (ANU). “We’ve only found evidence for 17 impacts older than 2.5 billion years, but there could have been hundreds. Asteroid strikes this big result in major tectonic shifts and extensive magma flows. They could have significantly affected the way the Earth evolved.”

This May, 2016, scientists have found evidence of a huge asteroid 20 to 30 kilometers across that struck the what is today Australia, creating a 400 kilometer-wide impact zone after breaking in two moments before it slammed into the Earth. The impact crater has long since disappeared. But a team of Aussie geophysicists has found the twin scars of the impacts – the largest impact zone ever found on Earth – hidden deep in the earth’s crust.

Tiny glass beads called spherules, found in north-western Australia were formed from vaporized material from the asteroid impact, said Glikson. “Large impacts like these may have had a far more significant role in the Earth’s evolution than previously thought,” Glikson said.

The exact date of the impacts remains unclear. The surrounding rocks are 300 to 600 million years old, but evidence of the type left by other meteorite strikes is lacking. “It’s a mystery – we can’t find an extinction event that matches these collisions. I have a suspicion the impact could be older than 300 million years,” he added.

“There are two huge deep domes in the crust, formed by the Earth’s crust rebounding after the huge impacts, and bringing up rock from the mantle below,” Glikson said. The two impact zones total more than 400 kilometres across, in the Warburton Basin in Central Australia. They extend through the Earth’s crust, which is about 30 kilometres thick in this area.

“The impact would have triggered earthquakes orders of magnitude greater than terrestrial earthquakes, it would have caused huge tsunamis and would have made cliffs crumble,” said Glikson, from the ANU Planetary Institute. “Material from the impact would have spread worldwide. These spherules were found in sea floor sediments that date from 3.46 billion years ago.”

About 3.8 to 3.9 billion years ago the moon was struck by numerous asteroids, which formed the craters, called mare, that are still visible from Earth “Exactly where this asteroid struck the earth remains a mystery,” Glikson said. “Any craters from this time on Earth’s surface have been obliterated by volcanic activity and tectonic movements.”

Glikson and Arthur Hickman from Geological Survey of Western Australia found the glass beads in a drill core from Marble Bar, in north-western Australia, in some of the oldest known sediments on Earth. The sediment layer, which was originally on the ocean floor, was preserved between two volcanic layers, which enabled very precise dating of its origin.

Glikson has been searching for evidence of ancient impacts for more than 20 years and immediately suspected the glass beads originated from an asteroid strike. Subsequent testing found the levels of elements such as platinum, nickel and chromium matched those in asteroids. There may have been many more similar impacts, for which the evidence has not been found, said Glikson.